Role of inducible defenses in the stability of a tritrophic system

Inducible defenses are a form of phenotypic plasticity that potentially modify direct interactions between various members of an ecological community, generating trait-mediated indirect effects. In this work, the hypothesis that inducible defenses increase the stability of tritrophic chains is tested, through the numerical analysis of a continuous-time model that discriminate between defenses affecting attack rate of predators, and defenses affecting predator handling time. In addition, discrimination between feeding costs of defenses affecting attack rate, and metabolic costs affecting feeding requirement for zero growth are considered. System stability was examined by computing dominant Lyapunov exponents, and through continuation routines of bifurcation points. Background parameter values were taken from two published studies. Our results show that a tritrophic system will generally be stabilized by the incorporation of inducible defenses and by their associated costs, but a number of new outcomes were obtained. Different long-term behavior is predicted if either one or two prey populations exhibit defenses. In the latter case, the defense of the basal prey dominates the dynamics. Handling time based inducible defenses exert a stronger stabilizing effect than attack rate based ones, but also impose a higher extinction risk for top predators. Inducible defenses in particular and trait-mediated indirect effects in general can be important sources of stability in natural systems.     

植食性昆虫适应植物防御反应的研究进展

在植物与植食性昆虫协同进化过程中,植物在不断完善其防御反应,同时植食性昆虫也在选择压下不断适应植物防御反应。植食性昆虫适应植物防御反应存在多样性。昆虫能够利用其唾液中的效应因子抑制或弱化植物防御反应,激活其肠道中的某些特异性蛋白阻断植物防御性次生代谢物的产生或者将其直接降解,以及通过其携带微生物间接抑制植物防御反应。此外,昆虫还能够通过产卵、虫害诱导植物挥发物、识别植物防御物质等方式适应植物的防御反应。本文综述了植食性昆虫如何利用各种效应因子适应寄主植物防御反应的研究进展。     

Salivary signals of European corn borer induce indirect defenses in tomato

Plants can recognize the insect elicitors and activate its defense mechanisms. European Corn Borer (ECB; Ostrinia nubilalis) saliva, produced from the labial salivary glands and released through the spinneret, is responsible for inducing direct defenses in host plants. Glucose oxidase (GOX) present in the ECB saliva induced direct defenses in tomato. By contrast, GOX activity in ECB saliva was insufficient to trigger defenses in maize, suggesting that host-specific salivary elicitors are responsible for inducing direct defenses in host plants. Our current study further examined whether ECB saliva can trigger indirect defenses in tomato. Relative expression levels of TERPENE SYNTHASE5 (TPS5) and HYDROPEROXIDE LYASE (HPL), marker for indirect defenses in host plants, were monitored. Quantitative real-time PCR analysis revealed that ECB saliva can induce the expression of TPS5 and HPL, suggesting that salivary signals can induce indirect defenses in addition to the direct defenses. Further experiments are required to identify different ECB elicitors that are responsible for inducing direct and indirect defenses in host plants.     

Changes in plant chemical defenses and nutritional quality as a function of ontogeny in <Emphasis Type="Italic">Plantago lanceolata</Emphasis> (Plantaginaceae)

Numerous empirical studies have examined ontogenetic trajectories in plant defenses but only a few have explored the potential mechanisms underlying those patterns. Furthermore, most documented ontogenetic trajectories in plant defenses have generally concentrated on aboveground tissues; thus, our knowledge regarding whole plant trends in plant defenses throughout development or potential allocation constraints between growth and defenses is limited. Here, we document changes in plant biomass, nutritional quality and chemical defenses for below- and aboveground tissues across seven age classes of Plantago lanceolata (Plantaginaceae) to evaluate: (1) partial and whole plant ontogenetic trajectories in constitutive chemical defenses and nutritional quality, and (2) the role of resource allocation constraints, namely root:shoot (R:S) ratios, in explaining whole plant investment in chemical defenses over time. Overall investment in iridoid glycosides (IGs) significantly increased, while water and nitrogen concentrations in shoot tissues decreased with plant age. Significant variation in IG content between shoot and root tissues across development was observed: allocation of IGs into root tissues linearly increased from younger to older plants, while non-linear shifts in allocation of IGs during ontogeny were observed for shoot tissues. Finally, R:S ratios only weakly explained overall allocation of resources into defenses, with young stages showing a positive relationship, while older stages showed a negative relationship between R:S ratios and IG concentrations. Ontogenetic trajectories in plant quality and defenses within and among plant tissues can strongly influence insect herbivores’ performance and/or predation risk; thus, they are likely to play a significant role in mediating species interactions.     

Non-ant antiherbivore defenses before plant-ant colonization in Macaranga myrmecophytes

We examined changes in the intensity of non-ant defenses of three myrmecophytic Macaranga species before and after the initiation of symbiosis with ants in a Bornean dipterocarp forest. The intensities of non-ant defenses at different growth stages of each Macaranga species were estimated by measuring the survival rate of larvae of the common cutworm, Spodoptera litura, when the larvae were fed on fresh leaves from seedlings (saplings) at three growth stages of each Macaranga species. In all species, the intensity of the non-ant defenses when seedlings had not yet received symbiont foundress queens was significantly higher than that after ant defense was well established. These results support the hypothesis that myrmecophytic Macaranga may defend themselves sufficiently via non-ant defenses before beginning symbiosis with ants and that the intensity of non-ant defenses may decrease as the symbiont colony size increases. We suggest that, where the status of myrmecophytism changes as plant–ant colonies grow, the decrease in the intensity of non-ant defenses which we detected after the establishment of ant colonies might generate an optimal allocation of metabolic cost to ant and non-ant defenses under resource limitations. We also measured leaf toughness, which is considered to be one of the most important agents of non-ant defenses against herbivorous insects, at different plant stages to assess its contribution to the change in the intensity of non-ant defenses after ant colonization. However, we found no evidence that changes in leaf toughness have a significant effect on the change in balance of the two antiherbivory mechanisms. Received: February 2, 2001 / Accepted: August 21, 2001     

Induced defense in Nicotiana attenuata (Solanaceae) fruit and flowers

Plants protect themselves against herbivory using a continuum of strategies, ranging from constitutive defenses to intermittent induced responses. Induced defenses may not provide immediate and maximum protection, but could be advantageous when continuous defense is either energetically or ecologically costly. As such, induced defenses in flowers could help defend relatively valuable tissue while keeping reproductive structures accessible and attractive to pollinators. Thus far, no one has demonstrated the efficacy of induced defenses against floral herbivores (florivores) in the field. Here we show that mechanical leaf damage in wild tobacco, Nicotiana attenuata (Solanaceae), reduced both flower and fruit herbivory in the field and that exogenous application of methyl jasmonate, a potent elicitor of induced responses, reduced both leaf and floral damage in natural populations. This result is consistent with a survey of damage in the field, which showed a negative relationship between leaf damage and flower and fruit damage. Although optimal defense theory predicts that induced defenses should be rare in reproductive tissues, owing to their high fitness value, our results suggest otherwise. Induced defenses in leaves and reproductive tissues may allow plants to respond effectively to the concomitant pressures of defending against herbivory and attracting pollinators.     

Colorado potato beetle manipulates plant defenses in local and systemic leaves

Herbivore microbial associates can affect diverse interactions between plants and insect herbivores. Some insect symbionts enable herbivores to expand host plant range or to facilitate host plant use by modifying plant physiology. However, little attention has been paid to the role of herbivore-associated microbes in manipulating plant defenses. We have recently shown that Colorado potato beetle secrete the symbiotic bacteria to suppress plant defenses. The bacteria in oral secretions from the beetle hijack defense signaling pathways of host plants and the suppression of induced plant defenses benefits the beetle’s performance. While the defense suppression by the beetle-associated bacteria has been investigated in local damaged leaves, little is known about the effects of the symbiotic bacteria on the manipulation of plant defenses in systemic undamaged leaves. Here, we demonstrate that the symbiotic bacteria suppress plant defenses in both local and systemic tissues when plants are attacked by antibiotic-untreated larvae.     

Are extrafloral nectaries efficient against herbivores? Herbivory and plant defenses in contrasting tropical species

植物用于防御的资源有限，而那些具有生物防御的物种，其叶片可能会较少地用于其他类型的防御。我们比较了机械防御能力较弱，但具有花外蜜腺的植物，与没有花外蜜腺但叶片更有韧性的植物是否更不容易被取食。我们通过分析文献数据，调查有无花外蜜腺的植物被取食的情况。在巴西南部的热带稀树草原植被中，我们采集了6种热带植物的叶片数据，并测定了被取食情况和比叶面积。我们进一步评估了有无花外蜜腺的物种之间以及植物之间被取食情况和比叶面积的差异。为了检验动物取食与叶片韧性之间的关系，我们对单株平均被取食情况和平均叶片韧性进行了回归。研究结果表明，植物表现出不同程度的叶片损伤，但没有蚂蚁防御的植物因被取食造成的叶面积损失最大。不同植物的机械防御水平也不相同。无花外蜜腺的植株更坚韧，比叶面积值更小。虽然无花外蜜腺的植物有更多的坚硬叶片，但这种机械防御不足以损害和/或减少食草动物的取食，这表明由蚂蚁形成的生物防御可能比用于与叶片适口性相关的机械防御更有效。     

Positive Effects of Plant Genotypic and Species Diversity on Anti-Herbivore Defenses in a Tropical Tree Species

Despite increasing evidence that plant intra- and inter-specific diversity increases primary productivity, and that such effect may in turn cascade up to influence herbivores, there is little information about plant diversity effects on plant anti-herbivore defenses, the relative importance of different sources of plant diversity, and the mechanisms for such effects. For example, increased plant growth at high diversity may lead to reduced investment in defenses via growth-defense trade-offs. Alternatively, positive effects of plant diversity on plant growth may lead to increased herbivore abundance which in turn leads to a greater investment in plant defenses. The magnitude of trait variation underlying diversity effects is usually greater among species than among genotypes within a given species, so plant species diversity effects on resource use by producers as well as on higher trophic levels should be stronger than genotypic diversity effects. Here we compared the relative importance of plant genotypic and species diversity on anti-herbivore defenses and whether such effects are mediated indirectly via diversity effects on plant growth and/or herbivore damage. To this end, we performed a large-scale field experiment where we manipulated genotypic diversity of big-leaf mahogany (Swietenia macrophylla) and tree species diversity, and measured effects on mahogany growth, damage by the stem-boring specialist caterpillar Hypsipyla grandella, and defensive traits (polyphenolics and condensed tannins in stem and leaves). We found that both forms of plant diversity had positive effects on stem (but not leaf) defenses. However, neither source of diversity influenced mahogany growth, and diversity effects on defenses were not mediated by either growth-defense trade-offs or changes in stem-borer damage. Although the mechanism(s) of diversity effects on plant defenses are yet to be determined, our study is one of the few to test for and show producer diversity effects on plant chemical defenses.     

Molecular and structural antioxidant defenses against oxidative stress in animals

In this review, it is our aim 1) to describe the high diversity in molecular and structural antioxidant defenses against oxidative stress in animals, 2) to extend the traditional concept of antioxidant to other structural and functional factors affecting the "whole" organism, 3) to incorporate, when supportable by evidence, mechanisms into models of life-history trade-offs and maternal/epigenetic inheritance, 4) to highlight the importance of studying the biochemical integration of redox systems, and 5) to discuss the link between maximum life span and antioxidant defenses. The traditional concept of antioxidant defenses emphasizes the importance of the chemical nature of molecules with antioxidant properties. Research in the past 20 years shows that animals have also evolved a high diversity in structural defenses that should be incorporated in research on antioxidant responses to reactive species. Although there is a high diversity in antioxidant defenses, many of them are evolutionary conserved across animal taxa. In particular, enzymatic defenses and heat shock response mediated by proteins show a low degree of variation. Importantly, activation of an antioxidant response may be also energetically and nutrient demanding. So knowledge of antioxidant mechanisms could allow us to identify and to quantify any underlying costs, which can help explain life-history trade-offs. Moreover, the study of inheritance mechanisms of antioxidant mechanisms has clear potential to evaluate the contribution of epigenetic mechanisms to stress response phenotype variation.     

Induced chemical defenses in invasive plants: a case study with Cynoglossum officinale L.

The ‘evolution of increased competitive ability’ (EICA) hypothesis is an extension of optimal defense theory and predicts that reduced pressure from insect herbivores in the introduced range results in evolution of reduced defenses in invading plant populations, allowing greater allocation of resources to competitive traits such as growth rate and reproduction. The EICA hypothesis considered levels of defensive chemistry to be fixed within a particular genotype. In this paper, we propose that if herbivory is reduced in the introduced range, but chemical defenses are inducible in response to herbivory, evolution of reduced defenses and any associated increase competitive ability should not occur. Rather, mean induced and constitutive levels of induced defenses should be similar in introduced and native ranges, but the variance about mean induced levels should be greater in the introduced range. This is predicted because induced levels will occur less frequently in the introduced range where herbivory is reduced, thereby insulating these levels from the stabilizing selection expected in the native range where induced levels occur more frequently. We conducted a preliminary study to examine this by comparing constitutive and induced concentrations of total pyrrolizidine alkaloids (PAs) from native (European) and introduced (western North America) populations of Cynoglossum officinale L. The mean constitutive and induced concentrations of PAs did not differ between continents, but the variability of the induced concentrations was significantly greater for plants from the introduced range. Although our study with C. officinale is provisional due to a small sample size, it supports our predictions for evolution of inducible defenses in introduced ranges where herbivore pressure is reduced. Most chemical defenses in plants have been found to be inducible, so similar patterns may occur widely. If so, this weakens the generality of EICA’s predictions concerning chemical defenses. The effects of inducible defenses should be considered in cross-continent comparisons of other invasive plant species.     

Poor phenotypic integration of blue mussel inducible defenses in environments with multiple predators

Aquatic prey encounter an array of threat cues from multiple predators and killed conspecifics, yet the vast majority of induced defenses are investigated using cues from single predator species. In most cases, it is unclear if odors from multiple predators will disrupt defenses observed in single-predator induction experiments. We experimentally compared the inducible defenses of the common marine mussel Mytilus edulis to waterborne odor from pairwise combinations of three predators representing two attack strategies. Predators included the sea star, Asterias vulgaris (= Asterias rubens ), and the crabs Carcinus maenas and Cancer irroratus . The mussels increased adductor muscle mass in response to cues from unfed Asterias (a predatory seastar that pulls mussel shells open) and increased shell thickness in response to unfed Carcinus, a predatory crab that crushes or peels shells. However, the mussels did not express either predator specific response when exposed to the combined cues of Asterias and Carcinus , and mussels did not increase shell thickness when exposed to cues from Cancer alone or any pairwise combination of the three predators. Shell closure or 'clamming up' did not occur in response to any predator combination. These results suggest that predator-specific responses to the Asterias and Carcinus are poorly integrated and cannot be expressed simultaneously. Simultaneous cues from multiple predators affect the integration of predator specific defenses and predator odors from functionally similar predators do not necessarily initiate similar defenses. Ultimately, the degree that prey can integrate potentially disparate defenses in a multiple predator environment may have ecological ramifications and represent a seldom explored facet of the evolution of inducible defenses.     

Qualitative effects of inducible defenses in trophic chains

By means of qualitative techniques we analyze the consequences of inducible defenses of species embedded in trophic chains on the community stability and responses of population equilibrium densities to press perturbations. Our results show that the inclusion of inducible defenses in trophic chains leads to profound changes in system dynamics. Inducible defenses increase the likelihood of instability, especially when exhibited by species of lower trophic levels. We obtained biologically reasonable feedback conditions that must be satisfied to ensure stability. Species responses to press perturbation are modified by inducible defenses and their associated costs in multiple ways. Many of the direct effects in the community are reinforced, while indirect effects are either weakened, if they propagate in a top–down direction, or are unaffected if they propagate from basal species. The dominant view of inducible defenses as a stabilizing force seems to be valid only within a biologically constrained parameter space.     

Warning displays in spiny animals: one (more) evolutionary route to aposematism

To date, theoretical or laboratory simulations of aposematic evolution in prey animals have focused narrowly on internally stored chemical defense as the source of unprofitability and ignore aposematic advertisement of physical defenses such as spines (and defensive hairs, claws, etc.). This has occurred even though aposematism in spiny animals has been recognized since the 19th century. In this paper we present the first detailed theoretical consideration of aposematism in spiny animals, focusing on questions of initial evolution, costs of display, and coevolution of displays with defenses. Using an individual-based evolutionary model, we found that spines (or similar physical defenses) can easily evolve without aposematism, but when spines do evolve, aposematic displays can also easily evolve if they help to make the prey animal distinctive and if they draw attention to the physical threat. When aposematic displays evolve, they cause reduced investment in costly spines, so that, in addition to signaling unprofitability, aposematic display may enhance the cost-effectiveness of antipredator defenses (one exception to this conclusion is if the display is itself as costly as the defense). For animals with stinging spines, combining physical and chemical defense, the evolution of aposematic display may lead to reduced investment in the toxin compared to the spine. This occurs because spines act as both secondary (repellent) defenses and as primary defenses (their own visible, honest advertisement), whereas internally stored toxins only (generally) act as repellent secondary defenses. We argue that conspicuous aposematism in spines functions as an attention-getting mechanism, whereas conspicuous aposematic display in purely toxic animals may be explained by signal reliability arguments. Finally, one (more) route by which aposematism may initially evolve is by spiny rather than purely chemically defended species, spreading to species with other forms of secondary defense as the signal becomes common.     

Global patterns of leaf defenses in oak species

Plant defensive traits drive patterns of herbivory and herbivore diversity among plant species. Over the past 30 years, several prominent hypotheses have predicted the association of plant defenses with particular abiotic environments or geographic regions. We used a strongly supported phylogeny of oaks to test whether defensive traits of 56 oak species are associated with particular components of their climatic niche. Climate predicted both the chemical leaf defenses and the physical leaf defenses of oaks, whether analyzed separately or in combination. Oak leaf defenses were higher at lower latitudes, and this latitudinal gradient could be explained entirely by climate. Using phylogenetic regression methods, we found that plant defenses tended to be greater in oak species that occur in regions with low temperature seasonality, mild winters, and low minimum precipitation, and that plant defenses may track the abiotic environment slowly over macroevolutionary time. The pattern of association we observed between oak leaf traits and abiotic environments was consistent with a combination of a seasonality gradient, which may relate to different herbivore pressures, and the resource availability hypothesis, which posits that herbivores exert greater selection on plants in resource-limited abiotic environments.     

Induction of extrafloral nectar depends on herbivore type in invasive and native Chinese tallow seedlings

Although induced defenses are widespread in nature, and a potentially important strategy used by invasive plants, it is unclear how induced defenses vary among populations and whether the intensity and duration of induced defenses depends on herbivore type. For invasive plants, low herbivore loads in their introduced ranges can lead to differences in herbivore defense compared to their native ranges, but we currently know little about how induced defenses vary among native and invasive populations. We conducted a greenhouse experiment to examine variation in one type of induced defense, extrafloral nectar (EFN) production, among native and invasive populations of Chinese tallow tree, Triadica sebifera. We experimentally manipulated herbivory from an exotic generalist scale insect, a native generalist caterpillar, both herbivores, or neither and then examined EFN production by Triadica. Damage from leaf-chewing caterpillars resulted in strongly induced EFN in both native and invasive populations while damage from phloem-feeding scales did not. Extrafloral nectar production and dissolved solute content peaked 4 days after caterpillar herbivory for both native and invasive populations. Number and proportion of leaves producing EFN, EFN volume and concentration of dissolved solutes were similar among native and invasive populations. These results suggest that selection for indirect defenses may be different than selection for other defenses in the introduced ranges of invasive plants, as constitutive and induced EFN production is retained in invasive populations.     

The ecology and evolution of inducible defenses

Inducible defenses are responses activated through a previous encounter with a consumer or competitor that confer some degree of resistance to subsequent attacks. While the importance of inducible resistance has long been known in host-parasite interactions, it is only recently that its importance has emerged in other natural systems. Although the structural defenses produced by invertebrates to their competitors and predators are by no means the same as an immune response triggered by parasites, these responses all share the properties of (1) specificity, (2) amplification and (3) memory. This review discusses the following ecological consequences and evolutionary causes of inducible defenses: (1) Inducible defenses render historical factors important in biological interactions and can affect the probability of individual survival and growth, as well as affect population dynamics of consumers in some circumstances. (2) Although the benefits of inducible defenses are often balanced by fitness costs, including reduced growth, reproductive output and survivorship, the role of costs and benefits in the evolution of inducible defenses is by no means clear. A more integrated approach would involve a multivariate analysis of the role of natural selection on the inducible characters of interest, their norms of reaction and correlated fitness characters. (3) The disproportionate representation of inducible, morphological defenses among clonal organisms may be due to both a higher rate of origination and enhanced selection to maintain these defenses in clonal taxa. (4) Inducible defenses should be most common when reliable cues are available, attacks by biological agents are unpredictable, and the fitness gains of defenses are balanced by the costs. An integrated approach to studying inducible defenses would thus combine mechanistic estimates of costs, population-level estimates of defense effectiveness, and genetic estimates of correlations between fitness and inducible characters. This will allow us to estimate rates of evolution in these phenotypically plastic threshold characters.     

Tree species diversity alters plant defense investment in an experimental forest plantation in southern Mexico

How plant species diversity affects traits conferring herbivore resistance (e.g., chemical defenses), as well as the mechanisms underlying such effects, has received little attention. One potential mechanism for the effect of diversity on plant defenses is that increased plant growth at high diversity could lead to reduced investment in defenses via growth–defense trade‐offs. We measured tree growth (diameter at breast height) and collected leaves to quantify total phenolics in 2.5‐year‐old plants of six tropical tree species (N = 597 plants) in a young experimental plantation in southern Mexico. Selected plants were classified as monocultures or as polycultures represented by mixtures of four of the six species examined. Tree species diversity had a significant negative effect on total phenolics, where polycultures exhibited a 13 percent lower mean concentration than monocultures. However, there was marked variation in the effects of diversity on defenses among tree species, with some species exhibiting strong reductions in phenolic levels in mixtures, whereas others were unresponsive. In addition, tree species diversity had no effect on growth, nor was the negative effect of diversity on chemical defenses mediated by a growth–defense trade‐off. These results demonstrate that tree diversity can alter investment in chemical defenses in long‐lived tree species but that such effect may not always be under strong control by plant endogenous resource allocation trade‐offs. Regardless of the underlying mechanism, these findings have important implications for predicting effects on consumers and ecosystem function.     

Tradeoff between physical and chemical defense in plant seeds is mediated by seed mass

Plants have evolved both physical and chemical defenses to make the nutrients of attacked organs difficult to access or more toxic to resist animal consumption or/and pathogen attack. Although it is intuitive that a tradeoff could exist between physical and chemical defenses because of finite defense resources, many studies have failed to detect this tradeoff. We hypothesized that tradeoff between physical and chemical defenses in individual organs was mediated by the total resource allocation to those organs. In this study, we tested whether a tradeoff between physical (i.e. fiber content, which has proved to be a good indicator of investment into seed coat) and chemical defenses (i.e. total phenolics, which are abundant chemical defenses in plant seeds) existed in plant seeds by using 163 common species collected from Xishuangbanna tropical forest, southwest China. Then we tested whether this tradeoff was mediated by seed mass which could be a potential proxy of total resource investment per seed. Among the 163 species, there was large interspecific variation in both total phenolics (from 0.01 to 20.52%) and fiber content (from 4.47 to 81.49%). Our results supported our hypothesis: negative relationships between physical and chemical defenses were much stronger among small seeds than among large seeds. Our study suggests that total resource acquisition must be considered when evaluating defense tradeoffs. However, it is usually extremely difficult to measure this resource acquisition variation, thus we suggest utilizing easily measured proxies of acquisition variation to quantify tradeoffs.     

In situ tests on inducible defenses in Dictyota kunthii and Macrocystis integrifolia (Phaeophyceae) from the Chilean coast

Numerous experimental studies have reported inducible defenses in macroalgae, but most of them have been conducted in laboratory environments where algae were maintained detached from the substratum and in artificial flow regimes. The results of those experiments might not reflect the natural situation, which can only be studied in situ. We examined whether the brown macroalgae Dictyota kunthii (C. Agardh) Greville and Macrocystis integrifolia (Bory) show inducible defenses following exposure to different grazing levels (direct, water-borne cues from nearby grazed conspecifics, presence of a non-grazing herbivore and natural grazing) in field experiments, striving to maintain natural conditions as much as possible. We measured palatability of algae after exposure to different grazing levels by using live algae and agar-based food containing non-polar extracts. M. integrifolia showed no induction of defenses (at least not of non-polar compounds), suggesting constitutive defenses, absence of defenses (tolerance) or use of another strategy to avoid herbivory. These results are similar to those from previous laboratory experiments. In D. kunthii, defense was induced after two weeks of direct grazing by amphipods under field conditions. Water-borne cues from nearby grazed conspecifics, presence of a non-grazing herbivore and natural grazing did not induce defenses. Induction of defense in response to direct grazing agrees with results from a previous laboratory study, but while indirect cues induced defenses in the laboratory, there was no measurable induced defense in the field. Probably chemical cues from grazers are diluted quickly in the field, not reaching concentrations that cause induction of defenses. This might be the reason why in some algae induction by direct grazing is a more important defensive strategy than induction by water-borne cues. The results from our study also suggest that laboratory experiments showing induced defenses in response to grazed neighbours or mere grazer presence need to be interpreted with caution.